Abstract

A total of fifteen experiments including morphological, molecular genetic and host
plant relationship studies were conducted to differentiate between two sympatric
populations of brown planthopper (BPH), Nilaparvata lugens, one from rice (Oryza
sativa) and the other from Leersia hexandra, a weed grass. The scatter plot based on
seven morphometric characters indicated that N. bakeri was totally an isolated
species. Insects with high esterase activities (usually caught off rice) and those with
low esterase activities (usually caught off L. hexandra) showed 6-8% overlapping
between the two populations of N. lugens. But scatter plot of the morphological
characters of stridulatory organs produced distributions that were almost nonoverlapping
indicating that BPH with high esterase activity usually caught off rice is
different from BPH with low esterase activity usually captured from L. hexandra.
Scanning electron micrographs showed some variations in different morphological
characters between individuals from the two sympatric populations of BPH but these
were not population specific. No heterogametic mating occurred in mate choice experiments. Crosses between the
two BPH populations from different host-plants showed some barriers for hybrid
production. Some genetic incompatibility may exist between the two populations.
After being tested for esterase activity, samples were analysed for six loci found to be
polymorphic at 95% criterion namely, Mdh, Idh, Pgm, Gpi, 6Pgd and A cp . The
genetic distance (average 0.182) and the existence of a diagnostic enzyme marker
(GPI) between rice and Leersia infesting populations indicated that both populations
are closely related but different species. The inheritance of GPI, IDH and MDH
isozymes were studied in families generated from mating individuals of two
sympatric populations of N lugens. These isozymes were controlled by three loci,
Gpi, Mdh and Idh, respectively. These loci were inherited in simple Mendelian
fashions. Thirty one bands from both short and long primer RAPD were able to be
tested for segregating ratios in two families of N lugens and they were found to be
inherited in simple Mendelian fashions. In the population genetic studies, two
diagnostic bands, one from short primer RAPO (OP003.7; 0.6Skb) and the other
from long primer RAPD (pehA#6.3; 1.00kb) were found to be present only in the
Leersia infesting populations of BPH. The DPGMA cluster analyses based on both
enzyme and RAPD markers showed that all the rice infesting populations of N.
lugens clustered together as a group. On the other hand Leersia infesting populations
of the same localities formed another distinct cluster. In host plant relationship
studies, rice plants were found best suited for the establishment of the rice infesting
population, and L. hexandra was a favourable host for the Leersia infesting
population A consideration of the evidence from studies on host plant relationships,
reproductive isolation, hybridization, morphometric variations, level of esterase
activity, existence of diagnostic isozyme and DNA level markers, genetic distance,
consensus tree and molecular variance between N. lugens with high esterase activity
usually caught off rice and N. lugens with low esterase activity usually caught off L.
hexandra suggested that both insect populations from Malaysia belong to closely
related sibling species. This information has practical implications in formulating
effective control measures against N. lugens which is a major pest of rice not only in
Malaysia but also throughout South East Asia, South Asia and Australia